In Vivo Effects of Reduced-Sodium Perilymph Perfusion on Hair Cell and Neural Potentials
To determine the functional significance of the sodium-transport mechanisms of the outer hair cells (OHCs) in vivo, the effect of reduced perilymphatic sodium on cochlear potentials was investigated in the guinea pig by perfusion of scala tympani with a modified artificial perilymph. The Na+ concentration of the artificial perilymph was reduced by almost 95% (from 150 mM to 8 mM) by substitution with choline, and resulted in an estimated 80% reduction in perilymphatic Na+ on perfusion through scala tympani. OHC function was assessed using Boltzmann analysis of the low-frequency cochlear microphonic (CM) and measurement of the high-frequency summating potential (SP) recorded at the round window. Compound action potential (CAP) thresholds and waveforms were monitored at multiple frequencies and the amplitude of the spectrum of the neural noise (SNN) in silence was measured as an indicator of spontaneous neural activity.